In elements of this kind, the cover glass and/or protective structure can have the function of protecting the device from dust and impurities and/or ensuring a certain environmental atmosphere, like, for example, a certain pressure, a certain humidity or a certain type of gas, within the optical device. At the same time, it should be possible to couple in and out a light beam and/or electromagnetic radiation. The cover glass may be deposited on the wafer level already in manufacturing, so-called wafer-level packaging, or may exemplarily also be deposited as a sealing in a packaging process.
Micromechanically produced chips or devices including optical functions here are, for example, scanner mirrors, so-called scanning gratings, bolometers, photodiodes and photodiode arrays, charge-coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) image sensors, display applications or light modulators. These chips and/or devices are to be protected against, for example, contamination by particles, against humidity or also high-energy radiation from the ultraviolet (UV) and the deep ultraviolet (DUV) radiation ranges or be operated under vacuum or certain inert gas conditions. Furthermore, the optical devices necessitate at least one optical interface realized by a window or protective structure which is transparent for the wavelength range necessary for the device.
There are a number of manufacturing methods for manufacturing optical devices of this kind including protective structures.
The diced chips can be packaged. At first, the individual chips or devices are produced by sawing, laser cutting or specifically breaking a wafer. Subsequently, the diced chips are bonded in respective standard or special packages. After that, electrical contacting of the device can be performed by means of wire bonding. Alternatively, the chip may, for example, on its back side, comprise a ball grid array comprising contact pads via which electrical contacting may be performed. Subsequently, the package may be sealed by applying a transparent cap which serves as a protective structure. In this method, before the actual packaging or capping, the chip may be tested on the wafer level so that only functional chips will be continued to be processed. However, the chips are separated from the wafer without any protection of the surface, for example by sawing or breaking, thereby making the process more complicated and potentially causing additional failures on the wafer level after the functional test. Another essential disadvantage of this solution is the usage of relatively expensive individual packages.
Alternatively, the chip may be capped by wafer bonding. The wafer including the optical devices and/or sensor/actor chips here may be connected to a second wafer, the so-called cap wafer such that a full-area cap results. The cap wafer here may exemplarily be a glass wafer for the necessary visible wavelength range or be made of silicon for the infrared wavelength range. If appropriate, a so-called spacer is used which ensures that there is a certain spacing between the wafer containing the optical devices and/or sensor/actor chips and the cap wafer. This may be necessary when mechanical elements of the sensor/actor wafer must not be limited in their movement. Exemplarily, a base wafer may be bonded to the back side of the sensor/actor wafer. This may, for example, be necessary when a vacuum is necessary for operation, the sensor/actor wafer, however, is perforated, but is to be vacuum-sealed. This method of wafer bonding for capping the chip is of advantage in that the chips are capped before dicing and are thus considerably less sensitive to the further dicing and processing procedure.
Another way of manufacturing optical devices including protective structures is using so-called pick & place machines, using which individual caps and/or protective structures may be placed onto a wafer with high position accuracy and precision. Using bonding layers, like, for example, glue or solder, a connection can be made between the sensor/actor wafer and the cap placed thereon. This method is of advantage in that the chips can be characterized on the wafer level before capping and caps will then only be placed on the functional chips. The functional chips specified for further processing will then, like in wafer bonding, be considerably less sensitive to the dicing and processing procedure. If appropriate, this method may be combined with a wafer bonding method for the back side of the sensor/actor wafer, i.e. of the wafer comprising the optical device.
In all the cases described, the transparent caps and/or protective structures are applied in parallel to the chip surface. The parallelity of the cap and chip surfaces generally is not a problem for purely optical sensors. However, if the electromagnetic radiation and/or the light is not only coupled in, but also out again, like, for example, in the case of light modulators or scanner mirrors, disturbing light reflections may occur on the protective structure and/or the cover glass due to the parallelity of the cap and chip surface. Anti-reflection layers on the top and bottom of the cap can reduce, but not eliminate completely, this effect. An example to be mentioned is a two-dimensional deflecting scanner mirror for image projection. By the two-dimensional deflection of the scanner mirror, a laser beam directed onto the scanner mirror is guided over an image field which corresponds to the operating range. The desired image results by modulating the laser intensity in dependence on the position of the laser spot. However, the laser beam, before impinging on the scanner mirror, is also reflected partly at the cover glass. If the scanner mirror is deflected symmetrically around its zero state, the residual reflection at the cap will cause a laser point in the image center of the operating range.
In order to illustrate the order of magnitude of this effect, it is assumed that the laser is not modulated, i.e. generates a maximally light image field. The laser intensity I is, for example, distributed to 640×480=307,200 image points. The result, assuming a one hundred percent transmission of the cover glass, is a mean intensity of I/307,200 for each image point. Assuming that the cap has an anti-reflection layer and thus a residual reflection of 1−99.9%=0.01%, an additional intensity for the image point in the center will be roughly I/10,000. This is about 30 times the intensity of the remaining image point and thus disturbing for an observer.
The schematic structure of a known micromechanical scanner mirror in a standard package comprising a glass cap will be described referring to FIG. 1. The optical device 1 is a scanner mirror 2. The scanner mirror 2 comprises a mirror plate 3 rotatable around an axis perpendicular to plane of the drawing. The scanner mirror 2 may exemplarily be connected to the base 11 of the package by glue and/or a glued connection 15. A coated glass cap 7, which may exemplarily be connected to the frame 9 by means of glass solder or by means of glue and, in particular, has the task of keeping contamination and particles away from the scanner mirror and the mirror plate 3, is deposited on the frame 9 of the package. An electrical connection to the package including the parts 11, 9, 7 can be made via contact areas or bond pads 13. The respective contacting and/or bond wires and contacts are not shown in FIG. 1 for purposes of simplification. If the mirror plate 3 in the undeflected state is arranged in parallel to the chip surface and the glass cap and the main beam path 5 of a light beam penetrates the transparent glass cap 7 and hits the mirror plate 3, the reflected main beam path 5a will result by the reflection of the main beam path 5 at the mirror 3. If the mirror plate 3 is deflected, as is indicated in the drawing by the plate 3b indicated in broken lines, the reflection of the main beam path 5 will result in the reflected main beam path 5c. Thus, the angle between the light beams 5a and 5c is double the deflection angle between the positions of the plates 3 and 3b. The case in which the plate 3 is deflected by the same amount in a direction opposite to 3b is not shown. This would result in a further main beam path and/or light beam such that the main beam path 5 would result in exactly the bisector of the angle between this light beam and the light beam 5a. Since an anti-reflection layer of the glass cap 7 exhibits a residual reflection, what results is a sub-beam path 5b. The latter is of a considerably lower intensity than the main beam path 5a and 5c, respectively, but has a disturbing effect in the application, as has already been shown before by the estimate for a projection display. Further multiple reflections caused by the reflection at the glass cap 7 and the plate 3 are not illustrated in FIG. 1.